Piezoelectric motors are frequently employed in applications which require non-ferrous, non-magnetic motion control, such as to drive the motion of MRI motion phantoms within a MRI system. In these applications, motors that generate torque from the interaction of large currents and permanent magnets are generally unsuitable and/or dangerous to use in the vicinity of the high magnetic field of MRI systems. Use of these types of motors, such as stepper motors, induction motors, and electromagnetic motors, results in undesirable interactions between the MRI magnetic field and the ferrous material required for motor function.
Piezoelectric motors are compatible and safe for use inside low and high strength MRI systems. This is because they are based on voltage driven piezoelectric transducers, which can be designed and built without the use of ferrous materials. Piezoelectric motors operate based on the material properties of piezoelectric materials, typically a polled ceramic or polymer. These materials are exposed to an extremely powerful electric field to polarize the ceramic or polymer material, inducing a permanent electric field bias within the material structure.
This permanent electric field bias of a piezoelectric material causes the material to react mechanically to an applied voltage across the material. The material reacts in a linear fashion. Consequently, piezoelectric motors are generally well suited to producing linear motion.
Currently available rotary piezoelectric motor assemblies translate the linear motion from piezoelectric motors to a rotary motion, but are limited to low speed and/or low torque applications. Such rotary piezoelectric motor assemblies typically use one or two linear motors arranged about a rotary stage bearing. The inherent speed and force limitations of piezoelectric motors requires the use of more than one motor when high speed and torque are required. Where two or more motors are used, two main challenges arise in operating the motors cooperatively, namely, resonance and dissonance between the piezoelectric motors. Generally, resonance stores energy within the motor and rotary stage system, which can be released in undesired ways, causing vibration when the motors are engaged. Dissonance, on the other hand, results from interference between the piezoelectric motors due to small differences in mechanical feedback induced to each motor, causing noise, vibration, and harshness and reducing the speed, torque, efficiency, and life of the rotary motor.
Accordingly, there is a need for a piezoelectric motor assembly that permits the efficient translation of linear motion from a plurality of piezoelectric motors into rotary motion, while minimizing mechanical drive assembly resonance and dissonance therebetween.